Assessment of microbial presence and activity in deep subsurface clay layers selected for radioactive waste disposal (PhD, Postdoc)

Over the past 20 years, radioactive waste microbiology has included a variety of analyses of disposal options for High Level (HLW) and Low/Intermediate Level (L/ILW) waste. Projects have covered a wide range of areas including: fundamental microbiology of geological formations; microbial tolerance to extreme conditions;

biodegradation of repository materials and the interaction of radionuclides with microbes and their by-products. The work has included field sampling, laboratory studies, natural analogue investigations and development of mathematical models.

Clay layers are frequently selected as natural or engineered barriers in the design of waste repositories, because of their excellent physico-chemical characteristics. At different locations, deep geological clay layers are currently investigated for the disposal of high-level (HLW) and medium level (MLW) radioactive waste. European examples are Boom clay (Mol, Belgium), Callovo-Oxfordian clay (Meuse/Haute Marne, France) and Opalinus Clay (Saint-Ursanne, Suisse).

To ensure the clays are able to maintain their beneficial barrier characteristics over extended periods of time, it is essential to investigate the processes that can have a significant impact on and the evolution of the key physico-chemical characteristic of the clay during the different disposal phases. This includes the bio-geochemical activity, due to the presence and activity of indigenous and/or foreign micro-organisms, as it can have detrimental effects on different safety aspects of radioactive waste disposal. The microbial presence and activity in the clay repositories can have potentially an impact on :

(ii)                Clay geochemistry, and thus affecting water-rock interactions (e.g. sorption/desorption equilibrium), radionuclide speciation, migration and transport (e.g. dissolution and complexation chemistry)

(iii)               Repository materials, via Metal corrosion (e.g. Microbial Influenced Corrosion (MIC) of metal canisters and over-packs); and Geo-mechanics (e.g. concrete deterioration, gas generation).

During the different disposal phases of a closed radioactive waste repository, micro-organisms will have to deal with truly extreme environmental conditions. Not only will micro-organisms have to cope with elevated levels of radiation but they will also be subjected to a combination of high temperature (70 – 80 °C), due to the heat output of HLW and extreme high, alkaline pH (12.5 – 13.5), due to the extensive use of cement-based materials (e.g. the Belgian Super Container concept). On the other hand, the high content of organic material in some clays (e.g. Boom Clay 3 – 5 %) and the huge amount of cellulose and nitrate in MLW could serve as a nutritional source for bacterial development.

The first aim of this project is to identify and characterize the population of micro-organisms that are capable of surviving in the vicinity of clay based radioactive waste repositories. Culture based techniques using a mixture of different substrates will be used to resuscitate specific populations and individual isolates. The isolates will be identified and phylogenetic analyses using a variety of genetic loci (e.g. rrn, apr, dsr, …) will be performed. In parallel a non-culture based metagenomic investigation, sequencing all genetic material present in the clay and clay water, will be undertaken. This metagenomic analysis will allow to gain insight of the genetic sequences and metabolic potential present in clay bacterial communities. State of the art bioinformatic-tools will be used to extract from these metagenomic data potential functional metabolic pathways that might be attributed to specific organisms. Ideally the result of this study should allow modeling and forecasting of the concentrations of metabolic end products that might be generated in a clay based radioactive waste repository.

Secondly, this project aims to assess the adaptation potential of the clay organisms to extreme conditions. Mixed natural microbial communities and lab cultures of purified strains (i.e. of the dominant species) will be subjected to successive rounds of high pH, high temperature, or a combination thereof. Surviving organisms will be further investigated at gene level by gene profiling and expression analysis.

The work will included field sampling and laboratory studies. Mainly the underground HADES Laboratory in the Boom clay of SCK•CEN will be used, but contacts and visits to microbial research groups of the Mont Terri Rock Laboratory (Switzerland) and of the Bure Underground Laboratory (France) are foreseen.